Multistage pump and manufacturing method

ABSTRACT

A multistage centrifugal pump having a divided forged and formed housing with a generally round and elongated pumping chamber defined within the housing, an integral multistage impeller assembly having a plurality of impellers dynamically balanced for rotation after final fabrication then rotatably mounted within the chamber, a plurality of interstage pump volute members each comprised of segments fitted between the impellers and around the impeller assembly and mounted in the housing within the chamber, and a method for manufacturing the pump.

United States Patent Blom 1 1 Jan. 21, 1975 [5 MULTISTAGE PUMP AND 2,578,617 12/1951 Watson ..415/199A MANUFACTURING METHOD 3,229,642 1/1966 Lobanofl' et a1. 415/199 A [75] Inventor: Carl J. Blom, Los Angeles, Calif. FOREIGN PATENTS OR APPUCATlONS 25,391 0/1905 Great Britain 415/199 A [73] Assgnee' 3;" Warm" 462,038 12/1949 Canada 415/199 A [22] Filed: N V- 1, 19 2 Primary Examinerl-lenry F. Raduazo [21] APP] No: 302,698 gaggle Agent, or Firm-Aubrey L. Burgess; John 0.

Related US. Application Data [63] cgntiinuatiion of Ser. No. 99,871, Dec. 21, 1970, [57] ABSTRACT a an one A multistage centrifugal pump having a divided forged 1521 U.S. c1 415/198, 415/199 A, 415/219 (3, and formed housing h a generally Found and 1"- 415/104 gated pumpmg chamber defined within the housing, [51] Int CL U F04! 29/42, F04d 17/08, F04d 17/12 an ntegral multistage impeller assembly having a plu- [58] new 0 Search 415/97, 98, 99 100 104, rality of lmpellers dynam1cally balanced for rotanon 415/105, 108 199 R 219, 199 A, 198 219 ztilgter tilmalbfabncaglmn ltliien zo tattablty mounted w1th1tn C e C am er, a ul'al y 0 m l'S age pump VOU e members each comprised of segments fitted between [56] References Cited the impellers and around the impeller assembly and mounted in the housing within the chamber, and a l 980 337 Z I I E PATENTS 415,199 A method for manufacturing the pump.

0 an er 2,108,786 2/1938 Bigelow et al. 415/198 5 Claims, 8 D g Figures /f/'" -fi l5 /2 I 6 M4 F5 mg 7 a2 50 v 1 1 1/52 72 1 1 0'2 94 54 mg as w w l i 1/ 72 e5 2 56 95 9 1 i i 546 7a 1Q. 1 I o1 1 la? 74 PATENTED JANZI I975 WEI 2 OF 5 INVENTOR 6424 J 560M PATIENTED I975 3.861.825

' INVENTOR @454 J 640M Q MH MULTISTAGE PUMP AND MANUFACTURING METHOD This is a continuation of application Ser. No. 99,871 filed Dec. 21, 1970, now abandoned.

BACKGROUND OF THE INVENTION This invention generally pertains to a multistage centrifugal pump (1) having a predictably homogenous and substantially flawless housing; (2) having structure providing easy and inexpensive replacement of the pump components which incur erosion and wear; and (3) having means whereby the pump impeller assembly may be finally fabricated with the impellers being heat shrunk on the pump shaft and the assembly being dynamically balanced before incorporation into the pump. Such a pump is particularly suited for service where high pressure, high temperature, dangerous and- /or noxious liquids are pumped. Examples of such services are boiler feed, refinery and chemical processes and power plants.

Presently used pumps of this kind generally are provided with a divided housing of cast material wherein the interstage volutes or diffusers are integrally cast into the housing. It is difficult to form castings which are predictably homogenous and free of internal flaws. For critical pumping service it is necessary to minutely inspect such castings, generally with radiographic techniques, and castings having voids or flaws either require repair as an additional remedial phase during manufacture of the pump housing or must be scrapped. Such castings require considerable lead or advance time in a manufacturing program and are relatively expensive.

A more recent technique is to form the divided housing as a more simple casting with integral interstage volutes or diffusers being formed separately. In this arrangement the pump impeller assembly is first assembled, then dynamically balanced, disassembled, reassembled with the volutes placed between the impellers, and finally the reassembled impeller assembly with interstage volutes is incorporated into the pump housing.

With this technique, most of the problems pertaining to castings remain along with the rather involved procedure of assembling, balancing, disassembling and reassembling the pump impeller assembly with the interstage volutes before incorporating the same into the pump.

Examples of present and previous construction for pumps of this kind are shown in U.S. Pat. Nos.

2,294,143 and 2,286,522.

Of these U.S. Pat. Nos. 3,229,642, 1,941,442, 1,998,754 and 2,286,522 disclose structure incorporating features which have similarities to features of the present invention.

OBJECTS AND SUMMARY OF THE INVENTION A principal object of this invention is to provide a pump having components requiring a minimum of radiographic inspection and a minimum of remedial work to such components to maintain acceptable quality standards during manufacture of the pump.

Another object of this invention is to provide a pump having components and subassemblies of consistently higher quality than comparable components in prior pumps.

Yet another object of the invention is to provide a pump having components which may be manufactured at substantially less cost than comparable components in prior pumps.

A further object of this invention is to provide a pump wherein less lead or advance time is required to manufacture stocks of initial roughed components for subsequent finishing and fabrication.

A further object of this invention is to provide a pump requiring a smaller inventory of replacement repair parts.

Another important object of this invention is to provide a pump which may be readily and easily repaired and overhauled with a minimum of tools and/or special equipment while remaining in a field installation.

These and other objects and advantages are attained in the multistage centrifugal pump and method of manufacture thereof disclosed herein including: a divided forged and formed pump housing having a generally round and elongated pumping chamber defined within said housing; a pump impeller assembly having a plurality of impellers serially mounted on a rotatable shaft and rotatably mounted with said housing within said chamber; said impeller assembly being dynamically balanced for rotation after final fabrication thereof; and a plurality of divided or segmented interstage pump volute members fitted around said impeller assembly and between said impellers and mounted with said housing and disposed within. said chamber.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a side elevational view of an exemplary pump of this invention as incorporated in service with a driving source and power train.

FIG. 2 is a longitudinal sectional view through the length of the pump housing, seals and portions of the pump shaft bearings.

FIG. 3 is an enlarged longitudinal section of a portion of the pump taken from just left of the center of the pump housing as shown in FIG. 2.

FIG. 4 is a transverse sectional view taken along the line 4-4 of FIG. 3.

FIG. 5 is a transverse sectional view taken along the line 5-5 of FIG. 3.

FIG. 6 is a transverse sectional view taken along the line 66 of FIG. 2.

FIG. 7 is a transverse sectional view taken along the line 77 of FIG. 2.

FIG. 8 is a transverse sectional view taken along the line 8--8 of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. 1 there is illustrated a pump installation generally indicated at 10 comprising a pump 12 connected through ,a rotary power coupling 14 to a gear unit 16 which in turn is connected through another rotary power coupling 18 to a power source 20.

Power source 20 is illustrated as an electric motor rated, for example, at 300 HP at 3,600 RPM. The pump as shown in the drawings may have a capacity, for example, of approximately 100 GPM (water) at a 2,600 PSI discharge pressure with a l530 PSI suction pressure when rotated at about 12,500 RPM through gear unit 16. As provided, the gear ratio of gear unit 16 would increase the 3,600 RPM output of power source 20 to the 12,500 RPM input for pump 12. In certain installations power source 20 can be a gas or a steam turbine, for example, in which event gear unit 16 would be provided of different gear ratio as required.

Pump 12 is equipped with mounting feet 30 mounted on pedestals 22 which project upwardly from a base 24. As shown, gear unit 16 and power source 20 are also mounted on base 24. Base 24 is bedded to a foundation 28 on a bed of grout 26.

Pump 12 includes a housing 32 comprising a lower case 34 and an upper case 36. Lower case 34 is equipped with a fluid outlet 38 terminating in a pipe flange 40 or other suitable fluid connection, and a fluid inlet 42 also terminating in a pipe flange 44 or suitable connection (FIGS. 2, 6, 7 and 8). Pump 12 also includes a pump shaft 46 comprising a portion of a pump impeller assembly, later described. Shaft 46 is axially supported and aligned on its driven end by a bearing assembly 48 which is mounted with bolts 50 to lower case 34 by means of a mounting bracket 52. The opposite end of shaft 46 is both axially and longitudinally supported and aligned by a bearing assembly 54 which is mounted by bolts 56 to lower case 34 by means of a mounting bracket 58.

A partial longitudinal section of pump 12 is shown in FIG. 2. As shown, lower case 34 and upper case 36 are formed to define a housing bore 60 when assembled. Cases 34 and 36 (FIG. 6) are assembled in predetermined alignment by means of at least two dowel pin means 62 such as the one shown. Cases 34 and 36 are retained in assembled condition by a multiplicity of cap screws 64, such as the one shown, or similar means such as studs and nuts.

The ends of bore 60 (FIG. 2) are closed by retainer covers 66 and 68. Retainer covers 66 and 68 are retained in assembled condition with cases 34 and 36 by means of a multiplicity of stud and nut assemblies 70. Bore 60 is sealed from the exterior of pump 12 by means of seal rings 72 which are mounted around covers 66 and 68 and disposed between such covers and cases 34 and 36 and also by means of a suitable gasket (not shown) or sealing surfaces between cases 34 and 36.

As shown, impeller shaft 46 is supported in axial positions within bore 60 by bearings 48 and 54. Rotary mechanical fluid seal assemblies 74 and 76 are mounted within covers 66 and 68, respectively, and about shaft 46 to seal shaft 46 in rotatable relation within bore 60. Seal assemblies 74 and 76 may be any of the types presently commercially available and suitable for the service in which pump 12 is placed. Pump shaft 46 forms the rotatable support component for an impeller unit 78 which includes a plurality of pump impellers 8090 mounted on shaft 46.

Mounted within bore 60 in fluid communication with inlet 42 and impeller 80 is a fluid suction guide member 92. Mounted within bore 60 between impellers 80 and 82, 82 and 84, 86 and 88, and 88 and 90, respectively, are segmented volute stage members 94, 96, 98 and 100, later described. An intermediate suction guide member 102 is mounted in bore 60 in fluid communication with an intermediate suction inlet 104 and impeller 90. A seal baffle member 106 is mounted in bore 60 between guide member 102 and cover 66. Mounted within bore 60 between impellers 84 and 86 is a segmented pump stage divider member 108, later described.

Guide member 92 and volute 94 define a first stage pump chamber 110 within bore 60. Volutes 94 and 96 define a second stage chamber 112. Volute 96 and stage divider 108 define a third stage chamber 114. Intermediate suction guide 102 and volute 100 define a fourth stage chamber 116. Volutes 100 and 98 define a fifth stage chamber 118 and volute 98 and stage divider 108 define a sixth stage chamber 120.

Fluid communication between third stage chamber 114 and fourth stage chamber 116 is provided through a cross-over conduit 122 connected to upper case 36 between a tapered bore 124 formed through case 36 between third stage chamber 114 and connected to upper case 36 in communication with intermediate suction inlet 104. Conduit 122 is connected into case 34 by means of socket welds, for example. Fluid communication is provided between sixth stage chamber and fluid outlet 38 through a discharge fluid conduit 126 connected to flanges 40 and to lower case 34 through a tapered discharge bore 128 formed through lower case 34 into chamber 120. As can be seen, a greater or lesser number of pumping stages may be readily provided if desired.

Divided retainer rings 130 (FIG. 3) are mounted about shaft 46 and within all the impellers such as 82 and 84 to retain such impellers in longitudinal position on the shaft. Keys 132 are mounted in accommodating slots formed in and between shaft 46 and the impellers to retain such impellers in arcuate position on the shaft.

Seen mounted within case 32 about shaft 46 in fluid communication with inlet 42 is fluid suction guide member 92 (FIGS. 2 and 6). Guide 92 is formed of two disc shaped parts joined in spaced apart concentric relation by means of a semicylindrical wall 134 and webs 136. As shown, guide 92 with cover 68 and bore 60 defines a seal chamber 138 in which fluid pressure remains the same as in a suction chamber 140 defined within guide 92. An annular flange projecting from guide member 92 is fitted into an accommodating recess formed in bore 60 to seal suction chamber 140 from first stage chamber 110 and to longitudinally position guide member 92 within bore 60. The primary purpose of webs 136 is to prevent rotation or swirling of fluids within suction chamber 140 when the pump 12 is in operation.

Mounted as previously mentioned (FIGS. 2 and 7) are intermediate disc shaped suction guide member 102 and disc shaped seal baffle member 106. Guide member 102 is equipped with a generally semicircular wall 142 and a webb 144. Guide member 102 and baffle member 106 define an intermediate suction chamber 146 within bore 60. Baffle member 106 and cover 66 form a second seal chamber 148 within bore 60. Annular flanges which project from guide member 102 and baffle member 106 are fitted into accommodating recesses formed in bore 60 to position such members within the housing and to provide fluid seals between the members and the housing.

Seal chambers 138 and 148 (FIG. 2) are joined in fluid communication through passages 150 and 152 provided in upper case 36 and an equalizing fluid conduit 154 connected into such passages by socket welds,

for example. Fitted within baffle member 106 and about shaft 46 is a seal bushing 156. A seal ring is mounted between shaft 46 and bushing 156 and a divided retainer ring and a key are mounted intermediate shaft 46 and bushing 156 to retain the bushing in position as do the rings and keys of the impellers. With the structure as described, rotary seals 74 and 76 are subjected equally only to the suction pressure of fluids entering the pump 12 without regard to increased stage discharge pressures within the pump.

As seen, an annular flange projecting around stage divider 108 is fitted into an accommodating recess defined in bore 60 to longitudinally position the divider in the bore 60 and to provide a fluid seal between the divider 108 and the bore 60.

Referring now to FIGS. 3 and 4, each of the impellers, as shown by impellers 82 and 84, is provided with a hub 158 shrink-fitted to shaft 46 and retained by ring 130 and key 132 as previously mentioned. The internal diameter of the hub for impellers 82 and 88 is slightly less than for impellers 84 and 86, and the diameter of the hub for impellers 80 and 90 is slightly less than for impellers 84 and 86. The purpose of the difference in diameters is to facilitate assembly of impeller unit 78. The shaft 46 has a corresponding reduction in diameter at corresponding hub locations. The difference in such diameter may be small, for example, in the order or 0003-0005 inch. Each impeller is provided with a hollow disc-shaped fluid discharge structure 160 extending from the hub which is composed of generally circular walls 162 joined by generally helical vanes 164. Extending axially from discharge structure 160 opposite hub 158 is an annular suction throat 166. Each hub 158 and throat 166 is received in close fitting without contacting relation within an accommodating adjacent bore as shown in the drawing and later described.

As seen in FIG. 2 with reference to FIG. 3, seal bushing 156 is received in very close relation within the bore of baffle member 106 and each of impeller hubs 158 and impeller throats 166 is received in similarly close relation within accommodating bores of an adjacently mounted member. Such close clearances are provided to restrict the flow of fluids under pressure from any specific chamber to an adjacent chamber. As seen in FIG. 3 each such accommodating bore has de fined therein a successive land and groove arrangement 168 which purpose is to provide a series of annular orifices causing successive pressure drops or reductions in the fluid passing through. Such arrangement serves to further restrict flow under pressure between adjacent chambers to a reduced rate permitting acceptable pump efficiency.

Impellers 80, 82 and 84 are generally the same in construction except that the hub of impeller 84 is threaded for a threaded connection 170 with impeller 86 which has an accommodating thread. Impellers 86, 88 and 90 are also generally the same in construction as mirror images of impellers 84, 82 and 80, except as required for threaded connection 170. The discharge peripheries of impellers 84 and 86 are of slightly different shapes than the remaining impellers to facilitate a radial fluid discharge rather than the longitudinal fluid discharge from the impellers between stages. Impellers of this general type are in common commercial use and further description herein is unnecessary.

As is inherent with multistage pumps of this kind. the fluid discharge from impellers 80 and 82 creates an axial force or thrust away from the driven end of shaft 46 and the discharge from impellers and 88 creates an opposing and substantially equal axial thrust toward the driven end of the shaft. Axial thrust as may be generated by impeller 84 is nulled by opposing thrust from impeller 86. Threaded connection 170, in addition to facilitating assembly of impellers 84 and 86 on shaft 46, provides longitudinal alignment between the connected impellers with respect to shaft 46 and further resolves any thrust created by such impellers. Any slight resulting thrust imbalance as may be created by fluid pressure on the annular area of seal bushing 156. etc. will be borne by thrust bearings incorporated in bearing assembly 54.

As seen in FIG. 3, stage divider 108 is provided in symmetrical divided segments which are assembled between and about impellers 84 and 86 after assembly and rotational balancing of impeller unit 78 and which may be retained in assembled condition by means of a fastener 172 which may be a cap screw, dowel pin or the like.

As seen in FIGS. 2 and 3, volutes 94 and 96 are the same in construction and are interchangeable. Volutes 98 and 100 are constructed in mirror image of volutes 94 and 96. Accordingly only volute 96 will be described with reference to FIGS. 3 and 5. Volute 96 is seen to be comprised of complementary segments 174 and 176 which may be retained in assembled relation by means of a cap screw arrangement 178. As assembled volute 96 defines a radially extending flange 180 which is fitted into an accommodating radial recess defined in bore 60. A lug 182 projects from flange 180 into a pocket deflned in lower case 34 to arcuately orient and position volute 96 in housing 32 upon assembly of pump 12. When assembled, volute 96 defines fluid passages 184 and 186 which are defined in part by a rib 188 defined by segment 174 terminating in a web 190 which terminates at the lands of groove arrangement 168 and by a similar rib 192 and web 194 defined by segment 176.

Further details of construction may be seen by taking FIG. 2 with reference to FIGS. 6, 7 and 8. FIG. 6 shows inlet passage 42 to be increased into multiple passages for substantially uniform entry of fluids into suction chamber 140. FIG. 7 shows passage 104 to be increased into multiple passages for substantially uniform entry of fluids into intermediate suction chamber 146. Discharge bores 124 and 128, as shown in FIG. 8, are provided as being tapered, or more properly, as being diverging and increasing in area for kinetic energy recovery of the fluids being pumped. Also shown in FIG. 8 is a fluid conduit 126 formed in a smooth long radius curve and connecting flange member 40 with lower case 34 in communication with discharge bore 128. Flange 40 is supported from housing 32 by a bracket 196.

METHOD OF MANUFACTURE Housing 32 is originated by forging two like metal billets or members, or by forging a single billet or member to be later divided for initial members, for lower case 34 and upper case 36. The term forging as used herein is intended to encompass the shaping of a metal member by application of substantial pressure or force. whereby the resulting member is homogenous in structure and free of flaws and internal deficiencies. Lower case 34 and upper case 36 are then formed; formed meaning the machining, grinding and/or welding, etc. necessary to form the bores, recesses and other surfaces as required and shown in the drawing. The inlet, outlet and conduits are formed as shown. Shaft 46 is formed from bar stock. Covers 66 and 68 may be originated by forging or casting and forming to the necessary shapes as shown in the drawing. The internal components of pump 12 are not subject to critical pressures and fluid tightness requirements and are considered as being expendable for replacement after becoming excessively worn from fluid erosion. These component elements such as the fluid guides, impellers, stage divider, volute members and seal baffle may be originated with simple castings which are then formed as disclosed. During fabrication the impellers are serially or successively placed on shaft 46, after each impeller is heated, with each impeller being placed in position with an appropriate retainer ring and key. Impellers 84 and 86 are first mounted with the remaining impellers being mounted in sequence. Each impeller is then allowed to cool and shrink tightly to the shaft. After the impeller unit 78 has been fabricated the unit is dynamically balanced in rotation as necessary by conventional balancing techniques. The divider member 108 and the volutes are then assembled about and between the impellers as shown, and the flow guides, seal baffle, caps, mechanical seals and bearings assembled in each end of shaft 46 as shown. The resulting assembly is placed in lower case 34 and the complete pump unit 12 then assembled as shown.

While only the preferred embodiment of the invention has been described and illustrated herein, it is to be understood that various other forms and embodiments of this invention may be made without departing from the spirit of the invention as set forth herein.

I claim:

1. An improved multistage centrifugal .pump manufacture comprising:

a. a divided forged and formed pump housing having a generally round and elongated pumping chamber defined within said housing; a pump impeller assembly having a plurality of impellers serially mounted in interference fitted relation on a rotatable shaft and mounted with said housing within said chamber;

a plurality of interstage pump volute members comprising jointed segments, said volute members fitted around said impeller assembly and between said impellers and removably mounted in said housing and disposed within said chamber, said volute members constructed and arranged for complete separation from said housing and said impeller assembly, while retaining said impellers mounted on the shaft, upon disjointure of said segments from one another and their movement transversely of said impeller assembly and away from their locations between said impellers to positions clear of the outer periphery of said impeller assembly, wherein retainer covers are mounted within said housing and about the ends of said shaft for enclosing said chamber, and wherein seals are mounted in said retainer covers and about said shaft and means to provide an equal fluid pressure differential across said seals.

2. The manufacture of claim 1 wherein means incorporating said housing, said impeller assembly and said volute members cooperate to balance said shaft against longitudinal thrust from fluid pressure.

3. An improved multistage centrifugal pump comprising: a divided pump housing having a generally round and elongated pumping chamber defined within said housing, said housing including a lower case and an upper case, releasable fastener means for releasably retaining said upper and lower cases in assembled relation, a rotatably mounted pump impeller assembly having a rotatable shaft and a plurality of impellers serially mounted on said shaft and disposed within said chamber, a plurality of ring-shaped interstage pump volute members disposed around said shaft and between said impellers, means for releasably mounting said pump volute members within said chamber and on said upper and lower cases, each of said interstage pump volute members comprising complementary segments, and second releasable fastener means for releasably joining said complementary segments in assembled relation, wherein said means for mounting said pump volute members within said chamber and on said upper and lower cases comprise circumferential radial flanges on said complementary segments and cooperating groovemeans in said upper and lower cases for receiving said flanges, and detent means on each of said volute members and complementary detent means on at least one of said cases for preventing rotation of said volute members in said cases.

4. An improved multistage centrifugal pump comprising: a divided pump housing having a generally round and elongated pumping chamber defined within said housing, said housing including a lower case and an upper case, releasable fastener means for releasably retaining said upper and lower cases in assembled relation, a rotatably mounted pump impeller assembly having a rotatable shaft and a plurality of impellers serially mounted on said shaft and disposed within said chamber, a plurality of ring-shaped interstage pump volute members disposed around said shaft and between said impellers, means for releasably mounting said pump volute members within said chamber and on said upper and lower cases, each of said interstage pump volute members comprising complementary segments, and second releasable fastener means for releasably joining said complementary segments in assembled relation, wherein said housing further comprises retainer covers at the ends of said housing and about the ends of said rotatable shaft, shaft seals mounted in said retainer covers and about said rotatable shaft, and pressure equalizing means for equalizing the fluid pressure differential across said seals.

5. An improved multistage centrifugal pump as defined in claim 4, wherein said housing, said impeller assembly and said volute members cooperate to balance said shaft against longitudinal thrust from the fluid pressure. 

1. An improved multistage centrifugal pump manufacture comprising: a. a divided forged and formed pump housing having a generally round and elongated pumping chamber defined within said housing; b. a pump impeller assembly having a plurality of impellers serially mounted in interference fitted relation on a rotatable shaft and mounted with said housing within said chamber; c. a plurality of interstage pump volute members comprising jointed segments, said volute members fitted around said impeller assembly and between said impellers and removably mounted in said housing and disposed within said chamber, said volute members constructed and arranged for complete separation from said housing and said impeller assembly, while retaining said impellers mounted on the shaft, upon disjointure of said segments from one another and their movement transversely of said impeller assembly and away from their locations between said impellers to positions clear of the outer periphery of said impeller assembly, wherein retainer covers are mounted within said housing and about the ends of said Shaft for enclosing said chamber, and wherein seals are mounted in said retainer covers and about said shaft and means to provide an equal fluid pressure differential across said seals.
 2. The manufacture of claim 1 wherein means incorporating said housing, said impeller assembly and said volute members cooperate to balance said shaft against longitudinal thrust from fluid pressure.
 3. An improved multistage centrifugal pump comprising: a divided pump housing having a generally round and elongated pumping chamber defined within said housing, said housing including a lower case and an upper case, releasable fastener means for releasably retaining said upper and lower cases in assembled relation, a rotatably mounted pump impeller assembly having a rotatable shaft and a plurality of impellers serially mounted on said shaft and disposed within said chamber, a plurality of ring-shaped interstage pump volute members disposed around said shaft and between said impellers, means for releasably mounting said pump volute members within said chamber and on said upper and lower cases, each of said interstage pump volute members comprising complementary segments, and second releasable fastener means for releasably joining said complementary segments in assembled relation, wherein said means for mounting said pump volute members within said chamber and on said upper and lower cases comprise circumferential radial flanges on said complementary segments and cooperating groove means in said upper and lower cases for receiving said flanges, and detent means on each of said volute members and complementary detent means on at least one of said cases for preventing rotation of said volute members in said cases.
 4. An improved multistage centrifugal pump comprising: a divided pump housing having a generally round and elongated pumping chamber defined within said housing, said housing including a lower case and an upper case, releasable fastener means for releasably retaining said upper and lower cases in assembled relation, a rotatably mounted pump impeller assembly having a rotatable shaft and a plurality of impellers serially mounted on said shaft and disposed within said chamber, a plurality of ring-shaped interstage pump volute members disposed around said shaft and between said impellers, means for releasably mounting said pump volute members within said chamber and on said upper and lower cases, each of said interstage pump volute members comprising complementary segments, and second releasable fastener means for releasably joining said complementary segments in assembled relation, wherein said housing further comprises retainer covers at the ends of said housing and about the ends of said rotatable shaft, shaft seals mounted in said retainer covers and about said rotatable shaft, and pressure equalizing means for equalizing the fluid pressure differential across said seals.
 5. An improved multistage centrifugal pump as defined in claim 4, wherein said housing, said impeller assembly and said volute members cooperate to balance said shaft against longitudinal thrust from the fluid pressure. 